PROJECT ABSTRACT Autism spectrum disorders (ASD) are a growing concern, with more than 1 in every 68 children affected in the US by the age of eight years. Complex interactions between genes and environmental factors are thought to contribute to ASD risk. Evidence is accumulating for a potentially large role in ASD etiology and/or morbidity for the early in-utero environment, including gestational nutrition. Maternal folic acid is one of the first modifiable factors identified to date with the potential to reduce the risk of ASD by ~40%. In addition to being essential for neurodevelopment, folate is critical for preventing apoptosis, and is involved in nuclear and mitochondrial DNA synthesis and repair. Folic acid is associated with reduced ASD risk at a time in early pregnancy when impaired mitochondrial function may influence processes highly dependent on energy, such as neurodevelopment. In addition, our work was first to show that children with ASD are more likely to have mitochondrial dysfunction (MD), mtDNA overreplication, and mtDNA deletions than typically developing children. To this end, we propose to leverage data and samples from mother-child pairs in a large NIH-funded cohort study to examine protection against mitochondrial abnormalities in placenta samples as a potential pathway for ASD prevention through maternal dietary and supplemental folate intake. Samples will be obtained from the Markers of Autism Risk in Babies: Learning Early Signs (MARBLES) study of >300 mothers who have a child with confirmed ASD and became pregnant with another child, in order to understand what influences the outcome of the younger sibling and to identify early markers of ASD. This is a significant advantage given that samples and data on demographic, lifestyle, health, interpregnancy interval, dietary, and other potentially relevant factors have been prospectively collected in this study since 2006, and children?s developmental assessments and clinically confirmed diagnoses will be available. This project will provide evidence on whether maternal and placental folate affects mitochondrial DNA and function, which then influence the developing child?s neurobehavioral outcomes including ASD. Findings will inform folic acid prevention strategies.